The present invention relates to electrostatic recording materials, which can be used for, for example, electrostatic plotters.
With the recent increase in use of computer aided design and the like, electrostatic plotters have come to be used in various applications such as the printing of drawings since they can output large format drawings at a high speed. Known electrostatic recording materials used in such electrostatic plotters typically have a conductive layer and a dielectric layer superimposed, in this order, on a support.
In electrostatic recording, a recording voltage is applied to a multi-stylus electrode to generate a gaseous discharge across a small gap between the multi-stylus electrode and a dielectric layer of the electrostatic recording material so that latent images can be formed on the surface of the dielectric layer, which latent images are subsequently developed with toner to produce visible images. Therefore, a small gap between a multi-stylus electrode and a dielectric layer is indispensable. Such a small gap is generally provided by introducing a pigment into the dielectric layer.
As the printing speed of electrostatic plotters has been increased and/or their resolution enhanced, printing defects such as "dropouts", which refers to omission of fine lines and small dots of printed areas, and "flares", which refers to generation of dots having a diameter three or five times larger than that of normal dots, have become more serious problems. Though various proposals have been made to eliminate these printing defects, conventional electrostatic recording materials are incapable of simultaneously solving the problems of "dropouts" and "flares" and hence are not fully satisfactory.
To solve the problem of "dropouts", it has been proposed to use a pigment with a Mohs' scale hardness of less than 5 in combination with a specific amount of another pigment with a Mohs' scale hardness of 5 or more (Japanese Patent Application Laid-open No. 2-250059). This technique may reduce "dropouts", but it cannot prevent the generation of "flares".
On the other hand, to solve the problem of "flares", it has been proposed to use a pigment with a Mohs' scale hardness of 3 or less and which has been subjected to a special surface treatment (Japanese Patent Application Laid-open No. 5-165226). This technique may reduce "flares", but it cannot prevent the generation of "dropouts".
Based on the foregoing prior art teachings, it had been expected that a pigment having a Mohs' scale hardness of 3 or less, subjected to a special surface treatment and combined with a proper amount of a pigment having a Mohs' scale hardness of 5 or more, might alleviate the both problems. However, contrary to that expectation, such a pigment tended to enhance the "dropouts" and the "flares" and proved to be impractical.
Therefore, the object of the present invention is to provide an electrostatic recording material which can simultaneously solve the problems of "dropouts" and "flares" and can output drawings with printed areas having a high density.